Fluidized bed combustion has become one of the most popular methods for burning solid fuels. Fluidized bed combustion uses a bed of sand as a grid, air distribution mechanism, fuel particle scrubber and thermal energy flywheel to burn solid fuel more cleanly and efficiently than other combustion systems. Combustion air is forced up through a bed of sand at high enough velocity to suspend the grains of sand. The volume of the sand expands and the sand appears to boil, bubble and churn. Hence, the sand is said to be xe2x80x9cfluidized.xe2x80x9d The sand is initially heated by an external source, usually a gas burner, until it is hot enough to ignite the fuel. For most solid fuels used in conventional fluidized bed combustion, the sand is pre-heated to about 800xc2x0 F. Once the sand is pre-heated and fuel introduced into the combustion chamber ignites, the burner can be shut off. The heat released by the burning fuel continues to heat the sand until equilibrium is reached between the cooler combustion air entering the sand and the burning fuel, typically about 1,500xc2x0 F. The incoming air forces the gaseous and solid reaction products of the burning fuel up through the combustion chamber as the fuel continues to burn, heating the gases to about 1,750xc2x0 F. These hot gases are passed through a boiler to make steam to produce electricity. Steam tubes are often immersed in the bed sand to extract heat to produce more steam while helping control the temperature of the sand. Gases exhausted from the combustion chamber are sometimes used to pre-heat the combustion air so that the combustion process begins at a higher temperature, improving efficiency.
A heat engine, such as a Stirling engine, uses heat applied externally to do work. In a Stirling engine, heat is applied externally to one end of a cylinder. The heat expands a working gas in the engine to push a piston and do work. A displacer then pushes the working gas to the cool end of the cylinder where it contracts to pull the piston and do more work. Heat engines are sometimes referred to as external combustion engines because heat is applied to the engine from an external source.